Acta Geophysica vol. 60, no. 3, Jun. 2012, pp. 858-873 DOI: 10.2478/s11600-012-0007-y ________________________________________________ © 2012 Institute of Geophysics, Polish Academy of Sciences Testing a Scaling Law for the Earthquake Recurrence Time Distributions Elisaveta MAREKOVA Plovdiv University “Paisiy Hilendarski”, Plovdiv, Bulgaria e-mail: eligeo@uni-plovdiv.bg Abstract The earthquake recurrence time distribution in a given space-time window is being studied, using earthquake catalogues from different seismic regions (Southern California, Canada, and Central Asia). The quality of the available catalogues, taking into account the completeness of the magnitude, is examined. Based on the analysis of the catalogues, it was determined that the probability densities of the earthquake recur- rence times can be described by a universal gamma distribution, in which the time is normalized with the mean rate of occurrence. The results show a deviation from the gamma distribution at the short interevent times, suggesting the existence of clustering. This holds from worldwide to local scales and for quite different tectonic environments. Key words: recurrence times, scaling law, universality, power law, earthquake catalogues. 1. INTRODUCTION Seismicity and earthquake triggering are highly complex phenomena in the spatial, temporal, and energy domains. The dynamics of these phenomena is not fully understood, because there is no complete knowledge of the physical processes occurring in the Earth’s lithosphere. The statistical properties of seismicity have been a subject of intensive interest for many years; e.g., one can mention Omori law (for the after- shocks, which defines the decrease in the number of earthquakes following Author copy